1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention relates to techniques for improving interconnect metallization performance in integrated circuits.
2. Description of the Related Art
In the manufacture of semiconductor integrated circuits (xe2x80x9cICsxe2x80x9d), well-known metallization techniques are used to interconnect devices on different levels of an IC chip. Generally, the performance of the interconnect metallization, or metallization line, (xe2x80x9cmetallization performancexe2x80x9d) involves providing proper conductivity, ease of etching of the interconnect material, minimizing electromigration, and minimizing capital investment and developmental effort in the metallization process.
Thus, in the design of any IC, strong consideration is generally placed on examining the degree of expected electromigration that may occur in view of a metallization line""s current carrying requirements. This is typically required because designers know that if too much electrornigration occurs in a given metallization line having a particular width, serious reliability-impacting voids may form. Accordingly, designers are commonly required to increase a metallization line""s width when high levels of current are anticipated, such as, for example, in power and ground bus lines. In certain circumstances, the designer is forced to make particular metallization lines exceedingly wide, just to prevent the possibility of excessive voiding from occurring. Widening metallization lines does, however, impose a cost penalty since this will require semiconductor chips to be larger than may be necessary to carry out the IC""s designed function.
Electromigration is commonly understood to be the result of an average current flow through a conductor. The flowing electrons transfer a fraction of their momentum to the metal atoms from a scattering process. This momentum transfer in turn causes a movement of the metal atoms (i.e., mass transfer) in the direction of electron flow. Therefore, the amount of momentum transfer, and resulting metal flow, increases with increasing current density. This flow of material is seldom uniform and regions of tensile stress develop where there is a net loss of material over time and regions of compressive stress develop where there is a net increase of material over time. The development of regions of tensile and compressive stress therefore create stress gradients. These stress gradients also cause a flow of metal since stress drives a flow of atoms from regions of high concentration (i.e., compressive stress) to regions of low concentration (i.e., tensile stress). For more information on electromigration and the degrading effects of electromigration, reference may be made to an article entitled xe2x80x9cEffects of W-Plug Via Arrangement on Electromigration Lifetime of Wide Line Interconnects,xe2x80x9d by S. Skala and S. Bothra, Proceedings of the International Interconnect Technology Conference, San Francisco, Calif., June (1998). This article is hereby incorporated by reference.
Electromigration voids are most commonly formed at the beginning of an interconnect line. This is believed to occur because electromigration degradation is more likely to stop when the sum of electromigration and stress is zero, which will more likely occur at the end of a line. Early observations of electromigration flow and its tendency to stop when a line is relatively short (e.g., a short distance to its terminating end) and continue when a line is relatively long (e.g., a long distance to its terminating end), was first reported by I. A. Blech. The behavior of electromigration defined in terms of the length of a metallization line has thus become widely referred to as the xe2x80x9cBlech effect.xe2x80x9d That is, when a metallization line is at least as short as a given Blech length for a particular width, electromigration voids will no longer form. For more information on Blech effect and Blech length, reference may be made to an article entitled xe2x80x9cElectromigration and Stress-Induced Voiding in Fine Al and Al-alloy Thin-Film Linesxe2x80x9d by C. K. Hu, K. P. Rodbell, T. D. Sullivan, K. Y. Lee and D. P. Bouldin, IBM Journal of Research and Development, Vol. 39, No. 4, July 1995, pp. 465-497. This article is hereby incorporated by reference.
Although the Blech length has been widely known, this concept is generally not applicable for many interconnect metallization lines and power buses because such lines are generally required to be longer than the Blech length in order to meet functional specifications. As a result, designers have continued to design certain metallization lines wider than necessary in order to prevent void formation which may introduce open circuits or complete functional failures.
Alxe2x80x94Cu alloys are one type of composition used for metallization lines in multi-level metallization for IC fabrication. However, if too much Cu is used in the alloy, the conductivity of the alloy decreases and it is more difficult to etch the alloy. At the other extreme, although pure Cu can be deposited with a dual damascene process, significant capital investment and developmental effort are required to provide pure Cu metallization in this manner. Pending reduction of such capital and developmental investment, efforts have been made to obtain improved metallization performance using Al-based alloys.
The Alxe2x80x94Cu alloy follows the classic solvus curve in which the solid solution of Al and Cu occurs at low weight percentages of Cu. Thus, the Cu appears primarily inside the grains of Al at such low weight percents of Cu, such that the grains are predominately composed of Al. At higher weight percents of Cu, the alloy composition shifts to the right of the solvus curve. The excess Cu now segregates to the grain boundaries between grains, in the form of Alxe2x80x94Cu precipitates.
In prior attempts to use Alxe2x80x94Cu alloys for improving interconnect metallization, Cu depletion from grain boundaries is a problem. This problem is schematically shown in FIG. 1A, where a magnified portion of an interconnect metallization 20 is shown including grains 21 (primarily of Al) separated by grain boundaries 22 (primarily of the precipitate Al2Cu). The grains 21 may be large as shown by grains 21L or may be small as shown by grains 21s. Electrical current is conducted through the interconnect metallization 20, and electron flow is depicted by a reference arrow exe2x88x92. Arrows E represent the electrons flowing between two adjacent grains 21 along a grain boundary 22. At the onset of electromigration, the electrons exe2x88x92initially carry atoms Cu, which are shown by dots and tiny circles in the grain boundary 22. The atoms migrating between exemplary grains 21-1 and 21-2 are shown diverging, which causes a net loss of material over time as the atoms migrate toward a group of the small grains 21s. As the atoms migrate to the right past the group of small grains 21s, they converge and accumulate as shown by the reference number 23, such that there is a net increase of material over time. The large grains 21L, which may grow depending on thermal history, can block flux along the interconnect metallization, and create flux divergence and voiding at the Cu-rich phase. Although proper heat treatment has been proposed as a solution to the goal of optimization of theta phase morphology, thermal cycles in backend fabrication processing and restrictions on elevated temperature processing preclude use of such proper heat treatment.
In more detail, FIGS. 1B and 1C show a magnified portion of FIG. 1A illustrating opposed grains 21 (e.g., grains 21-1 and 21-2) separated by the grain boundary 22. Arrows CU depict Cu atoms migrating away from the grain boundary 22 under the action of the electron exe2x88x92flow, resulting in Cu depletion from the grain boundary 22. Because the Cu atoms are heavier than the Al atoms, there is a time period (xe2x80x9cincubation timexe2x80x9d, or xcex94t; FIG. 5) during which the Cu atoms resist being moved by the electron exe2x88x92flow. Then, once the Cu atoms have been depleted along a length equal to the critical length xe2x80x981cxe2x80x99 from the grain boundary 22, the Al atoms begin to move away from the grain boundary 22 by the same electromigration process. However, the migration of the Al atoms is rapid. As a result, the Cu and Al atoms migrate to the end of the interconnect metallization 20, and as described above, may result in Al failure, which is a failure of the metallization 20. Such failure can be a serious problem where a void is formed just below a via. This, of course, can cause the entire IC device to fail working within the design specifications. As such, electromigration failures can be early in the life of the IC or may result after a period of use.
As mentioned above, a more important aspect of electromigration is that Al migration occurs not only from within the grain boundaries 22 upon Cu depletion from the grain boundaries, but also from the grains 21 adjacent to the grain boundaries 22. FIG. 1C shows a case where the Cu migration is followed by the rapid Al migration. Because the Alxe2x80x94Cu solvus curve indicates that there is very little Cu in solid solution within the Al, the very low weight percent of Cu in the grains 22 is not effective to prevent, or delay, the migration of the Al atoms from the grains 21 into the grain boundaries 22, such that voids 24 are formed in the grains 21. The voids 24 cause the Alxe2x80x94Cu alloy to have increased resistance, which decreases the performance of the devices, and can result in the failure of the IC.
In view of the foregoing, there is a need for an improved interconnect metallization alloy composition, and for methods for improving interconnect metallization performance, whereby the rapid onset of Al failure due to Al migration from the grains is significantly delayed, as compared to that occurring in Alxe2x80x94Cu alloys.
Broadly speaking, the present invention fills these needs by providing an improved composition in the form of an Alxe2x80x94Cuxe2x80x94Zn alloy for interconnect metallization, and methods for providing an Alxe2x80x94Cuxe2x80x94Zn alloy for interconnect metallization. The invention also fills the need for reducing the effects of Cu depletion from grain boundaries (and the resulting rapid onset of Al failure) by providing grains in an interconnect metallization that are a Zn-rich Alxe2x80x94Zn alloy. Thus, early electromigration failure is reduced. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, a composition, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a semiconductor device is described as including a substrate and a plurality of interconnect metallization lines defined over the substrate. Each of the interconnect metallization lines is provided with an electromigration-impeding composition including a percentage by weight of aluminum, a percentage by weight of copper, and a percentage by weight of zinc. In more detail, the percentage by weight of zinc may be less than about 4 in solid solution in Al at 100 degrees C., which is a substantial increase in the Zn content over the about 0.5 weight percent of the Cu content in Alxe2x80x94Cu alloys. As an example, the percentage by weight of Zn may range between about 1 and 2. In a more detailed embodiment, the lines may have an electromigration-impeding composition including a solid solution of Al and Zn, wherein the solid solution is in the form of grains, and the grains are bounded by grain boundaries. The composition further includes a precipitate of Al and Cu defined in the grain boundaries. Electromigration of the Al from the grain boundaries occurs and tends to cause the Al to electromigrate from the grains. The percentage by weight of Zn is selected to both define the solid solution with Al and to impede the electromigration of the Al from the grains.
In another embodiment, a method of making an interconnect metallization layer over a substrate is described as including depositing a metallization composition over the substrate. The composition includes an Al component, a Cu component, and a Zn component. Of course, the composition can also include trace impurities such as oxygen, iron, nickel, etc. The percentage by weight of the Zn component is less than about 4, and preferably ranges between about 1 and 2 percent by weight.
In yet another embodiment, a semiconductor device having improved interconnect metallization performance is described as including at least one metallization layer, the metallization layer having grains bounded by grain boundaries. The grains include a solid solution of predominantly Al and Zn and the grain boundaries include a precipitate of predominantly Al and Cu such that electromigration tends to deplete the Al from the grain boundaries and tends to cause migration of the Al from the grains. The Zn is effective to improve interconnect metallization performance by impeding the migration of the Al from the grains. The layer includes a percentage by weight of the Al; a percentage by weight of the Cu; and a percentage by weight of the Zn. The percentage by weight of Zn is less than about 4, such as a percentage by weight of Zn ranging between about 1 and 2. The percentage by weight of the Zn is selected to both define the solid solution with Al and to impede the electromigration of the Al from the grains without substantially increasing the resistivity of the metallization layer.
In the use of the Alxe2x80x94Cuxe2x80x94Zn alloy as a metallization layer for interconnect metallization, the Alxe2x80x94Cuxe2x80x94Zn alloy with the Zn in solid solution exhibits improved reliability. The presence of the larger, heavier Zn atoms delays the migration of the Al from the bulk grains into the grain boundaries. Any depletion of the Al existing within the grain boundaries would occur notwithstanding the Zn in the alloy, but such depletion is not significant enough to cause voiding. Studies indicate that a high percentage (e.g., 2 weight percent) of Zn in the Alxe2x80x94Cuxe2x80x94Zn alloy remains in solid solution in the grains, such that the Alxe2x80x94Cuxe2x80x94Zn alloy maintains superior electromigration resistance independently of the history of heat treatment. In addition, the Zn added to the Alxe2x80x94Cu alloy improves the mechanical strength of the interconnect metallization layers, and hence the reliability of the interconnects. Also, the Alxe2x80x94Cuxe2x80x94Zn alloy as a metallization layer for interconnects can be fabricated without a significant reduction in conductivity as compared to the Alxe2x80x94Cu alloy. These benefits result while reducing the negative impacts on ease of etching. For example, sputtering during etching can be used to remove excess Zn, although in some cases the addition of Zn to the Alxe2x80x94Cu alloy may require slightly more sputtering at higher power to remove the Zn residue.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.